The present invention relates generally to the field of semiconductor manufacturing and more specifically to three dimensional semiconductor memory devices and methods of making thereof.
During the fabrication of a semiconductor device, various lithographic masks are used to form structures on a substrate. Each lithographic step has its own mask. For critical levels, cost per mask is approximately $300,000 and increasing. Consequently, creating a mask set is very expensive. Moreover, creating a mask set for an experimental run can be prohibitively expensive. For example, fabricating eight levels of memory using a traditional unique mask for each design layer requires 24 critical masks in a back end of the line (BEOL) structure (i.e. metal interconnects). A mask set merely for the interconnects costs over seven million dollars.
A four layer three dimensional one time programmable (OTP) memory device typically consists of four layers of individual memory cells stacked on top of a device driver layer that is used to read and write the individual memory cells. Each individual cell has an associated word line and bit line. The word lines and bit lines are uniquely connected to the device driver layer by a chain of zias.